Yes, the ride becomes very harsh, to point of being almost painful each time you go over a bump. After the bump there is rapid, violent bouncing, not the slow wallowing you get with failed shocks on a normal set-up. Also, if you push down on the rear corner, like a standard shock absorber test, you will find it very hard.
At the time they fail, the level of hydraulic fluid drops dramatically and you may see air bubbles, or a 'froth' in it.
They are underneath the section of floor just behind the back seats (middle row if yours is a 7 seater) as Michael says. Just remove the four screws (can't remember if there are 4 or 6 and its too early to go out and have a look!)
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)